Rate convertor

ABSTRACT

Embodiments of the invention may be used to implement a rate converter that includes: 6 channels in forward (audio) path, each channel having a 24-bit signal path per channel, an End-to-end SNR of 110 dB, all within the 20 Hz to 20 KHz bandwidth. Embodiment may also be used to implement a rate converter having: 2 channels in a reverse path, such as for voice signals, 16-bit signal path per channel, an End-to-end SNR of 93 dB, all within 20 Hz to 20 KHz bandwidth. The rate converter may include sample rates such as 8, 11.025, 12, 16, 22.05, 24, 32 44.1, 48, and 96 KHz. Further, rate converters according to embodiments may include a gated clock in low-power mode to conserve power.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional application62/051,599, filed Sep. 17, 2014, entitled RATE CONVERTER, the contentsof which are incorporated herein by reference.

FIELD OF THE INVENTION

This disclosure is directed to signal processing, and, morespecifically, to a system for converting data samples at a first rate todata samples at a second rate.

BACKGROUND

In general, conventional rate converters include two major computationblocks, as illustrated in FIG. 1. A first block is a conversion ratetracking loop that determines the ratio between an input sample rate andoutput sample rate. Once the ratio is determined, the conversion ratetracking loop generates the corresponding input sample index for eachoutput sample.

The second computation block is the sample interpolator. The function ofthis block is to interpolate the input data sample and to generate theoutput data sample with the real value index. One problem withconventional rate converters is that they create aliasing from theresampling, and it is relatively difficult to produce an output signalhaving a high Signal-to-Noise Ratio (SNR) and having a relatively flatfrequency response within standard frequency ranges for audio signals.

Embodiments of the invention address this and other limitations of theprior art.

SUMMARY OF THE INVENTION

Embodiments of the invention may be used to implement a rate converterthat includes: 6 channels in forward (audio) path, each channel having a24-bit signal path per channel, an End-to-end SNR of 110 dB, all withinthe 20 Hz to 20 KHz bandwidth. Embodiment may also be used to implementa rate converter having: 2 channels in a reverse path, such as for voicesignals, 16-bit signal path per channel, an End-to-end SNR of 93 dB, allwithin 20 Hz to 20 KHz bandwidth.

The rate converter may include sample rates such as 8, 11.025, 12, 16,22.05, 24, 32 44.1, 48, and 96 KHz.

Further, rate converters according to embodiments may include a gatedclock in low-power mode to conserve power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional rate converter.

FIG. 2 is a block diagram for a conversion rate tracking loop of a firsttype of rate converter.

FIG. 3 is a block diagram for a conversion rate tracking loop of a rateconverter according to embodiments of the invention.

FIG. 4 is a graph showing passband behavior for anti-alias filtersaccording to embodiments of the invention.

FIG. 5 is a graph illustrating total wrapped aliases for all anti-aliasfilters according to embodiments of the invention.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F illustrate frequency responses for 1×,1.5×, 2×, 3×, 4×, and 6× filters according to embodiments of theinvention.

FIG. 7 is a block diagram of an interpolator portion of a ratecontroller according to embodiments of the invention.

FIGS. 8A-8B, 9A-9B, 10A-10B, 11A-11B, 12A-12B, 13A-13B, 14A-14B, aregraphs illustrating performance differences between an old rateconvertor and an example rate convertor according to embodiments of theinvention.

DETAILED DESCRIPTION

For comparison, FIG. 2 is a block diagram for a conversion rate trackingloop of a first type of rate converter, while FIG. 3 is a block diagramfor a conversion rate tracking loop of a rate converter according toembodiments of the invention.

In general, with reference to FIG. 3, inputs for a rate converter blockinclude an input audio sample, an input audio channel id, a signal thatthe input sample is ready, and a signal that indicates that the audiodestination is ready to take the next sample. In particular, the inputaudio samples are packed into one word for all the audio channels, whichis time multiplexed for all channels. The input audio channel id tellswhich channel the sample belongs to. The input sample ready signalsignifies that the next audio sample is ready, while the output sampleacknowledgement signifies that the audio destination is ready to takethe next sample. Other standard interfaces signals like clk, reset,enable, etc. may be present, as well as one or more control signals froma Direct Memory Address. Finally, there may be one or more register bankaccess signals.

There are also several output signals. An output audio sample includes asingle word with all of the audio channels output from the converter. Itis time multiplexed for all channels. The output audio channel id tellswhich channel the sample belongs to. An output sample ready signalsignifies that the next audio sample is ready. An input sample acksignifies that the converter is ready to accept the next signal from theaudio source. Also, a register bank access signal may be output.

In general, with reference to FIG. 3, the rate tracking loop tries toestimate the ratio between an input sample rate and output sample rate.In addition, it generates the input sample index that corresponds toevery output sample.

Both input and output sample rates may be jittery. The estimated samplerate ratio should be stable enough to yield a high SNR, and also shouldmaintain a stable average value for the sample index. Ideally, formaximum performance input buffers should not overflow or underflow, andaudio latency variation should be minimized.

In operation, initially, an estimate of drate (decimation rate) isobtained by either a user setting, or by measuring input and out samplerates. The estimated value is stored in drate reg, which is a registerthat stores the decimation rate.

The tracking loop circuit includes one or more different “gears.” In oneembodiment there are four gears. The difference between the gears is thevalue for the gain elements G1, G2 and G3. The gain values for each ofthe gear levels are programmed in registers and can be changed byfirmware. The gear control decides when to move the gear up or down. Thechange is controlled by two factors. In some embodiments, the converterstays in each gear for a minimal amount of time before switching toanother gear. Also in some embodiments, this minimal time doubles foreach higher gear. In one embodiment, the rate converter of FIG. 3 movesto a higher gear when the absolute value of error and the change slopeof error are both under certain thresholds. Likewise, if the absolutevalue of error or the change slope is above the thresholds, the trackingloop would move down the gear.

Table 1 includes default values for tracking loop parameter registersaccording to embodiments of the invention

TABLE 1 Gear 0 1 2 3 Log₂(G1) −3 −5 −7 −9 Log₂(G2) −7 −9 −11 −13Log₂(G3) −6 −8 −10 −12 Log₂(gear up abs) 9.5 7.5 5.5 N/A Log₂(gear upslope) 8 6 4 N/A Log₂(gear down abs) N/A 10.5 8.5 6.5 Log₂(gear downslope) N/A 9 7 5

The tracking loop of FIG. 3 converges with the default values. In someembodiment it converges within 1 second before the SNR is above 100 dB,and reaches final lock with the next few seconds. The final trackingyields a latency variation of less than 100 ns. The overall SNR impactis also small. More than 120 dB SNR can be reached.

In a particular implementation of embodiments of the invention, theindex error is sampled at 24 MHz to get time accuracy that is way belowthe sample period. The average by 2¹⁴ enables all of 2nd order phaselock loop and the IIR in error smoothing to operate at 1.5 KHz clock,which allows a very low current draw. The sample index buffer has 28bits. With one input sample counts as 2²³, that gives enough room for 16samples. Since the input buffer is only 16 samples more than the filterrequirement. It is enough to cover all possible cases. The drateregister contains 41 bits. That is 13 more LSB than the sample indexbuffer. The 13 LSBs may be a conservative number, but it costs verylittle in area and current. The error IIR buffer contains 45 bits. Itadds 13 LSBs to the input from index error average. Again, 13 LSBs maybe a conservative number, but it costs very little in area and currentsince no multipliers are required. The operation frequency is 1.5 KHz.Dithering is done at reducing drate from 41 bit to 28 bit before adds tosample buffer.

Compared to the rate converter illustrated in FIG. 2, the rate converterof FIG. 3 is much more efficient. The rate converter of FIG. 3 includesa simple 1st order IIR filter, while the converter of FIG. 2 has fourmore complex low pass filters. The simple 1st order IIR filter iscertainly much simpler and costs a lot less area and current. Eachfilter has different gain and different spectral/impulse response. Allfour filters are running all the time. The gear shift decides whichfilter result to pick. The average is over 2¹⁰ samples instead of 2¹⁴.

In one embodiment, the gear selection of the new rate converter dependsonly on time. For each gear level, it stays for a fixed amount time. Thestay time for gear level n+1 is twice as the time for level n. The timeto stay in each gear depends on the phase lock quality. Differentinitial conditions and sample conversion ratios decides how long tostay. A threshold based decision is a good implementation method.

There is no 2nd order tracking loop. Instead, the drate output is dumpedinto the register four times during the operation of the rate converter.

The first time is at beginning of rate conversion and it is set to theinitially estimated drate.

Each time gear changes up, the current drate is dumped into a register.

The sample index buffer is not adjusted for every output sample, butinstead it is adjusted when the accumulative increase is above one inputsample.

Introducing the 2nd order tracking loop is the biggest change forperformance compared to the converter of FIG. 2. The converter of FIG. 2always converges, but when it converges, the sample index is not 0. Thatcauses some variance in audio latency. If the data is dumped too often,it causes divergence. If the amount of change is reduced each time, thenit becomes a 2nd order tracking loop. If it is updated every time theIIR gets a new value, it behaves as the 2nd order tracking loop of FIG.3.

The older design of FIG. 2 updates the sample index buffer every timethe accumulation across the input sample, which is unnecessary complex.The design of FIG. 3 does not update according to the same schedule.

While the above description has been focused on rate tracking,embodiments of the invention additionally include a new design for theother major block of a rate converter, the sample interpolator.

Theoretical Background

In theory, the approach is to apply a continuous time filter withcertain frequency response to a discretely sampled input and thendiscretely resample it at another sample rate.

The frequency response of such a filter should reject the aliasgenerated by the discrete time input samples as much as possible whilemaintaining the audio passband as much as possible.

For implementation simplicity, embodiments of the invention use a filterlength of 80 times input sample period. For example, let's suppose thefilter value is f(t) for −40T_(s)<t≦40T_(s). Let the input samples bes_(n), the output sample s_(m)′=Σ_(k=−40) ⁴⁰s_(└mr┘+k)·f((mr−└mr┘+k)·T_(s)) where r is the output sample to inputsample ratio.

It is conventionally difficult to get the values of filter f. The filtercan be pre-computed and stored, but the memory requirement is very big.To achieve 120 dB alias rejection would require more than a millionentries stored, which is an impractically large number. Instead, thewhole 80 sample period may be broken into many pieces and given apolynomial for each piece to approximate the continuous time filter. Oneembodiment includes a 3rd order polynomial over 640 time pieces. Thatselection provides an alias less than 120 dB lower than signal for theentire audio pass band.

The way to generate such polynomials is first generate a vastlyoversampled filter with the desired frequency response and length. Then,within each of the 640 time pieces, the points of the filter are fit toa 3rd order polynomial with minimum square error. The performance ofsuch polynomials can be verified by computing 30 points in each timepiece and look at the overall frequency response.

FIG. 4 is a graph showing passband behavior for anti-alias filtersaccording to embodiments of the invention.

From the graph, it can be seen that the passband ripple for 6×decimation is 0.1 dB, for 4× decimation is 0.01 dB and even lower forother filters.

FIG. 5 is a graph illustrating total wrapped aliases for all anti-aliasfilters according to embodiments of the invention. This figure showsthat the total alias for 6× decimation is between −97 dB to −100 dB. Thetotal alias for 4× decimation is about 120 dB. The total alias for 1×filter is around −130 dB up to 10 KHz; then it goes up to −120 dB at 16KHz and to −112 dB at 20 KHz.

For all other filters, the alias rejection varies between −130 dB to−125 dB. There are occasional strays going up to −122 dB.

FIGS. 6A-6E illustrate individual frequency responses for 1×, 1.5×, 2×,3×, 4×, and 6× filters according to embodiments of the invention. Thehumps on the 1× filter and 1.5× filter is a result of the polynomialapproximation. For the other filters, since the cutoff frequency is muchlower, the polynomial approximation error is under the noise floor.

FIG. 7 is a block diagram of an interpolator portion of a ratecontroller according to embodiments of the invention. Although notillustrated, the multiplexers (muxes) are controlled by state machine.

In operation, one operation cycle is performed on each output sample. Inone embodiment, each operation cycle goes through 80 samplescorresponding to the filter size. For each of the 80 samples, first iscoefficient generation, and then the multiplier and adding over oneinput sample for each audio channel. When the whole computation is over,the output sample is stored in the 2 sample FIFO buffer.

In the following example, let N be the number of audio channels.

Each operation cycle takes (N+3)*80 cycles.

Let k be the sample index between 0 and 79.

The coefficient ROM is read in the following cycles:

-   -   For cycle k*(N+3)+1, the coef ROM reads out the 3rd order        coefficient and stores it to coef buffer;    -   For cycle k*(N+3)+2, the coef ROM read out the 2nd order        coefficient and passes it to the adder;    -   For cycle k*(N+3)+3, the coef ROM read out the 1st order        coefficient and passes it to the adder;    -   For cycle k*(N+3)+4, the coef ROM read out the 0th order        coefficient and passes it to the adder.

The coefficient buffer is updated during cycles k*(N+3)+1 to k*(N+3)+4.It takes input from coefficient ROM in cycle k*(N+3)+1 and takes inputfrom multiplier accumulator in other cycles.

The input RAM is read during cycles k*(N+3)+5 to k*(N+3)+N+4. Note thatthe last cycle of input RAM access overlaps with first cycle ofcoefficient ROM access. But this does not cause any problem.

The output accumulator is reset to 0 at cycle 0 and is updated duringcycles k*(N+3)+5 to k*(N+3)+N+4.

The multiplier selector takes LSB of sample index during cyclesk*(N+3)+2 to k*(N+3)+4 and takes input RAM during cycles k*(N+3)+5 tok*(N+3)+N+4.

The adder selector takes coefficient buffer during cycles k*(N+3)+2 tok*(N+3)+4 and takes output accumulator during cycles k*(N+3)+5 tok*(N+3)+N+4.

Let x be the LSB of sample index. The operation being done are:

Cycle k*(N+3)+1 gets c′=c₃

Cycle k*(N+3)+2 gets c′=cx+c₂=c₃x+c₂

Cycle k*(N+3)+3 gets c′=cx+c₁=c₃x²+c₂x+c₁

Cycle k*(N+3)+4 gets c′=cx+c₀=c₃x³+c₂x²+c₁x+c₀. That is the coefficientapplied over all audio channels.

Cycle k*(N+3)+j, 0≦j<N, gets b_(j)′=b_(j)+c·a_(j,k). Here b_(j) is theoutput accumulator for channel j and a_(j,k) is the input sample k forchannel j.

At the end of (N+3)*80 cycles, the output accumulators are dumped intothe output buffer and ready for output over output strobe.

Implementation Details

In an example implementation, for each filter, there are 640 timeintervals. Each contains a 3rd order polynomial. Therefore, 2560 wordsare needed for each filter coefficient ROM. However, symmetry reducesthe coefficient ROM. Since f(−t)=f(t), we only need to store half of thefilter polynomials. That is, 1280 words per filter coefficient ROM.

There is one input buffer and one output buffer for each audio channel.Each input or output buffer contains two 24 bit words. It serves as aFIFO to temporally store the input or output sample before written byinput RAM or output to the next block.

The input RAM contains 96 words of 24 bit width for each audio channel.In the 96 words, 80 are used for the anti-aliasing filter. The remaining16 are used for possible sample jitter and input/output rate mismatchbefore the phaselock loop completely locks.

The address gen generates the access address for the input sample RAMand the coefficient ROM. Suppose the input buffer start address is i,the sample index is a·2²³+b·2²⁰+c when an operation cycle starts. Thenthe kth sample index generated is (a+i+8+k)mod 96. The coefficient ROMaddress is 32·(a+k)+4·b+j, here j means the coefficient order of thepolynomial.

The multiplier is a 24 bit by 24 bit signed multiplier. The adder is a28 bit adder. The output accumulator is 28 bits too. Rounding isperformed before storing to the output sample buffer.

Compared to the operation of the interpolator that operates inconjunction with the rate tracker of FIG. 2, the major differences withthe interpolator are:

The conventional multiplier is a pipelined multiplier that takes 12cycles. The multiplier in the interpolator of FIG. 7 is a single cyclebooth multiplier come from synthesizer.

In the old interpolator x² and x³ must be computed because of the 12cycle multiplier latency. In the old interpolator, the new coefficientis computes as c₃x³+c₂x²+c₁x+c₀ while the new interpolator compute it as((c₃x+c₂)x+c₁)x+c₀ which does not need to compute x² and x³.

Each operation cycle of the old interpolator requires 92N+288 cycles and80N+240 cycles in the interpolator of FIG. 7. That means a higher clockrate is required in the old interpolator.

The old interpolator only has two filters, one for full bandwidth andthe other for half bandwidth. The new design has 6 filters, supporting1×. 1.5×, 2×, 3×, 4×, and 6× decimation. That practically allows anyrate to any rate conversion. Also, the old interpolator has 2560 wordsfor each filter and the new interpolator only has 1280 words for eachfilter.

The old rate converter computes all 80 coefficients before applyingthem. Therefore, it needs 80 word coefficient RAM. The new convertercomputes one coefficient and apples it to all of the channels beforegoing to the next one. Therefore, it only needs one word to store thecoefficient.

The impacts of the change include: the single cycle multiplier has abouthalf area and a third current compare to the pipelined multiplier; notrequiring x² and x³ saves area and current too; and a lower clock ratemeans less current.

More filters take more area which is a tradeoff, but it makes the designmore flexible and is able to handle all rate to all rate conversion.Also, the total words is 7680 words for the new design compared to 5120words for the old one.

Not requiring the coefficient RAM makes a big difference in area andcurrent.

FIGS. 8A and 8B are graphs that illustrate amplitude response and SNRfor similar rate conversions comparing an existing rate controller(labeled in the graphs as “old filter”) to a rate controller accordingto embodiments of the invention (labeled in the graphs as “new filter”).

FIGS. 9A and 9B are graphs that illustrate amplitude response and SNRfor 1.5× decimation comparing an existing rate controller (labeled inthe graphs as “old filter”) to a rate controller according toembodiments of the invention (labeled in the graphs as “new filter”).

FIGS. 10A and 10B are graphs that illustrate amplitude response and SNRfor 2× decimation comparing an existing rate controller (labeled in thegraphs as “old filter”) to a rate controller according to embodiments ofthe invention (labeled in the graphs as “new filter”).

FIGS. 11A and 11B are graphs that illustrate amplitude response and SNRfor 3× decimation comparing an existing rate controller (labeled in thegraphs as “old filter”) to a rate controller according to embodiments ofthe invention (labeled in the graphs as “new filter”).

FIGS. 12A and 12B are graphs that illustrate amplitude response and SNRfor 4× decimation comparing an existing rate controller (labeled in thegraphs as “old filter”) to a rate controller according to embodiments ofthe invention (labeled in the graphs as “new filter”).

FIGS. 13A and 13B are graphs that illustrate amplitude response and SNRfor 6× decimation comparing an existing rate controller (labeled in thegraphs as “old filter”) to a rate controller according to embodiments ofthe invention (labeled in the graphs as “new filter”).

FIGS. 14A and 14B are graphs that illustrate amplitude response and SNRfor interpolation rate conversion comparing an existing rate controller(labeled in the graphs as “old filter”) to a rate controller accordingto embodiments of the invention (labeled in the graphs as “new filter”).

From inspection of FIGS. 8A-14B, it is seen that, for most of the cases,a rate coder according to embodiments of the invention ha have 1˜2 dBhigher SNR compare to the old rate coder. The exceptions are, for 6×decimation, the new rate coder has 95 dB˜100 dB SNR while the old ratecoder has more than 10 dB higher SNR. This is because the new coderprovides very tight alias rejection compared to the old decoder.Similarly, for 4× decimation, the new coder has about 4 dB lower SNR,which is also due to increased computation for tight alias rejection.

When the input rate is very close to integer multiple of output rate,the alias falls right on the signal. In these cases, the old and newcoders have similar SNRs. That is because the alias falls almost exactlyon the signal and cannot be distinguished.

Embodiments of the invention provide: up to 8 channels of audio with asample rate<=48 KHz with a 48 MHz clock rate, and coverage of all rateto all rate conversion if the decimation rate is not more than 6×.

Embodiments of the invention may be incorporated into integratedcircuits such as sound processing circuits, or other audio circuitry. Inturn, the integrated circuits may be used in audio devices such asheadphones, sound bars, audio docks, amplifiers, speakers, etc.

Also, although embodiments of the invention have been described usingfunctional blocks, the block may be implemented in any physicalembodiment, as is known in the art. For example blocks may beimplemented in application specific integrated circuits (ASICs), FPGAsor other programmable firmware, software running on a specializedprocessor, software running on a general purpose processor, or anycombination of the above.

Having described and illustrated the principles of the invention withreference to illustrated embodiments, it will be recognized that theillustrated embodiments may be modified in arrangement and detailwithout departing from such principles, and may be combined in anydesired manner. And although the foregoing discussion has focused onparticular embodiments, other configurations are contemplated.

In particular, even though expressions such as “according to anembodiment of the invention” or the like are used herein, these phrasesare meant to generally reference embodiment possibilities, and are notintended to limit the invention to particular embodiment configurations.As used herein, these terms may reference the same or differentembodiments that are combinable into other embodiments.

Consequently, in view of the wide variety of permutations to theembodiments described herein, this detailed description and accompanyingmaterial is intended to be illustrative only, and should not be taken aslimiting the scope of the invention.

What is claimed is:
 1. A rate converter implemented in an audioprocessing circuit, the rate converter comprising: an input forreceiving audio data sampled at an input sample rate; a sample indexaccumulator for determining a sample index for each output audio sampleat an output sample rate different from the input sample rate; an erroraccumulator for determining and outputting an index error; a secondorder phase lock tracking loop for estimating a ratio between the inputsample rate and the output sample rate; a gear controller configured toadjust gain values for application to the audio data during conversionbetween the first sample rate and the second sample rate, the adjustedgain values applied within the tracking loop based on the index erroroutput of the error accumulator; and a sample interpolator configured toapply a continuous time filter with a predetermined frequency responseto resample the audio data from the input sample rate to the outputsample rate.
 2. The rate converter of claim 1, further comprising aplurality of filters, each configured to provide a different impulseresponse to the audio data during rate conversion.
 3. The rate converterof claim 2, in which the plurality of filters are infinite impulseresponse (IIR) filters.
 4. The rate converter of claim 2, in which thegear controller is configured to adjust gain values and provide impulseresponses to the audio data during rate conversion by selecting a filterfrom the plurality of filters for application to the audio data.
 5. Therate converter of claim 4, in which the gear controller selects a newfilter when an absolute value of the index error and a change of slopeof the index error are both under a threshold.
 6. The rate converter ofclaim 4, in which the gear controller selects a new filter when anabsolute value of the index error and a change of slope of the indexerror are both above a threshold.
 7. The rate converter of claim 4, inwhich the gear controller selects a filter for a fixed amount of time.8. The rate converter of claim 7, in which the plurality of filters areordered, and in which the fixed amount of time doubles for each higherorder filter.
 9. The rate converter of claim 1, in which the continuoustime filter is approximated by dividing a sample period into pieces andproviding a polynomial for each piece.
 10. The rate converter of claim1, in which the sample interpolator computes a single coefficient andapplies the single coefficient to all channels.
 11. The rate converterof claim 1, in which the sample interpolator employs a single cyclemultiplier.
 12. The rate converter of claim 1, in which the sampleinterpolator processes a plurality of samples of audio data according tofilter size by: generating a coefficient, applying a multiplier, andadding an input sample for each audio channel.
 13. The rate converter ofclaim 1, in which the input audio data is received as part of aplurality of input audio samples packed into one word for all audiochannels.
 14. The rate converter of claim 12, in which input channelidentifiers (IDs) are received to indicate which audio channel eachinput audio sample belongs to.
 15. The rate converter of claim 1, inwhich the rate converter is configured to receive an input sample readysignal indicating when a next sample of audio data is ready forconversion.
 16. The rate converter of claim 1, in which the rateconverter is configured to output a sample acknowledgement indicatingthat an audio destination is ready to take a new sample of audio data.17. The rate converter of claim 1, in which the rate converter isconfigured to estimate a decimation rate by measuring the input samplerate and the output sample rate.
 18. The rate converter of claim 17, inwhich the gear selector selects a filter for application to the audiodata based on the decimation rate.
 19. The rate converter of claim 18,further comprising a decimation rate register for storing the decimationrate.